Comparison of Structural and Electronic Absorption Properties of Poly (di-n-hexylsilane) and Poly (di-n-butylsilane)

Structural studies of polysilanes in a solution and in the solid state are presented. In the case of poly(di-n-hexylsilane) (PDHS), the conformation of the Si-backbone has been shown to be all-trans in the solid state. The uv-absorption shows a reversible thermochromic transition for the solid, during heating to 42C, corresponding to a partial disordering of the chains. PDHS also undergoes a thermochromic order-disorder transition in solution. 

In contrast, the chain conformation of poly(di-n-butylsilane) (PDBS) has been demonstrated to be a 7/3-helix, and to exhibit only a very limited and gradual thermochromic transition in the solid state. In solution, PDBS does display a reversible and sharp thermochromic transition, analogous to that of PDHS evidenced by a red-shift in the uv-absorption at -40C. 

By precipitating the PDBS from cold, dilute solutions, the conformation responsible for the low energy uv-absorption in solution has been isolated in the solid state and shown by uv-absorption, solid-state NMR and x-ray diffraction to differ from that of the 7/3 helical form. The solution precipitated polymer irreversibly converts to a somewhat disordered analog of the 7/3- helix as the temperature is raised.